There are many catalysts and catalyst preparations known which are useful in the conversion of aromatic or aliphatic hydrocarbon feedstocks to maleic anhydride by partial oxidation with air. Typically used feedstocks are benzene, butane and butenes, for which specific oxidation catalysts have been developed, mostly for fixed-bed catalysis operations. With known catalysts in fixed-bed catalysis operations running at relatively high space velocities (typically at 2500 hr.sup.-1), maleic anhydride yields of about 75 percent of theory are possible from benzene feedstock, and about 50 percent or less from butane. Typical of these catalysts are catalytic materials comprising a mixture of components selected from compounds of molybdenum, molybdenum oxides, vanadium, vanadium oxides, phosphorus and phosphorus oxides.
In U.S. Pat. No. 3,864,280 to Schneider, for example, there is disclosed a vanadium-phosphorus mixed oxide catalyst for use in oxidation of butane to maleic anhydride, which catalyst is prepared in a non-aqueous medium by reacting hydrogen chloride gas with vanadium pentoxide, followed by reaction with orthophosphoric acid. The Schneider patent discloses as the active ingredient of the catalyst a material designated as the "B-phase", which material is characterized by major X-ray diffraction pattern CuK.alpha. d-spacings of 6.3, 4.8, 3.9, 3.13, 2.98 and 2.65 angstroms.
U.S. Pat. No. 3,907,707 to Raffelson et al. discloses phosphorus-vanadium-oxygen catalysts for fluid bed conversion of butane to maleic anhydride, which catalyst is prepared by contacting a pentavalent vanadium compound with a trivalent phosphorus compound in an aqueous medium to form a catalyst precursor containing tetravalent vanadium. The catalyst precursor is heated in air at a temperature of 350.degree. C. to 600.degree. C. until about 20 to 80 atomic percent of the vanadium has been converted to pentavalent vanadium.
U.S. Pat. No. 3,977,998 to Freerks et al. discloses a process for preparing phosphorus-vanadium-oxygen catalysts for use in catalytic oxidation of butane to maleic anhydride wherein a vanadium compound and a phosphorus compound are heated in aqueous solution in the presence of an oxalic acid reducing agent to reduce pentavalent vanadium and then maintain the vanadium species in its tetravalent state. The recovered catalyst precursor material is then calcined at a temperature of 350.degree. C. to 600.degree. C. until 20 to 95 atomic percent of the vanadium has been oxidized to pentavalent vanadium.
British Pat. No. 1,538,031 of Monsanto Company discloses a process for preparing a phosphorus-vanadium-oxygen catalyst useful in catalytic oxidation of butane to maleic anhydride. The catalyst is derived by adding vanadium pentoxide slowly to a refluxing mixture of phosphoric acid and oxalic acid in water, then refluxing the mixture for 72 hours at 97.degree. C., and thereafter evaporating the mixture to dryness overnight at 120.degree. C. Precursor compositions of the catalysts are characterized by either of two X-ray diffraction patterns, one having d-spacings of 7.50, 5.79, 5.25, 4.17, 3.81, 3.63, 3.44, 3.40, 3.08, 2.81, 2.77 or 2.31 A, and one having d-spacings of 6.70, 5.68, 3.34, 3.16 and 3.09 A.
Belgian Pat. No. 867,189 of Imperial Chemical Industries Limited discloses a promoted solid oxide composite of vanadium and phosphorus prepared by reacting a vanadium compound with phosphoric acid in an aqueous solution of a strong acid to form a catalyst precursor, followed by evaporating the solution to provide a mixture of compounds including a "B-phase" and a compound having the structure .alpha.-VOPO.sub.4 as determined by X-ray diffraction. A component present in the dried mixture and characterized as detrimental to the catalyst is described by the structural formula VO(H.sub.2 PO.sub.4).sub.2.
A monograph entitled "Selective Oxidation of Butene into Maleic Anhydride" [K. Zeebot et al., Ukran. Khim. Zhur. 43, 842 (1977)] describes a butene catalyst of the "V/P-mica" group having a composition H.sub.x (H.sub.2 O).sub.m [(V.sup.+4 O).sub.x (V.sup.+5 O).sub.1-x PO.sub.4 ], wherein "x" may have a value of zero to 0.95.